1. Field of the Invention
The present invention is concerned most generally with metals and alloys of metals having added thereto metalloids which enhances the preparation of metal powders made from the modified metal or metal alloys and improves the properties of the metal powders when used in the forming of or net-shape forming of articles from the metal or alloys by processes such as supersolidus sintering. More particularly the present invention is directed to palladium-silver-copper alloy compositions of increased hardness containing metalloids which improve certain characteristics of the alloys. The overall hardness of the basic alloy is increased, the supersolidus temperature is reduced thereby aiding in the supersolidus sintering process, the initial grain size in powder particles of the alloy is reduced and upon atomization and rapid solidification the powder metal produced thereby has optimum particle and grain size for use in supersolidus sintering for the net-shape forming of articles made using the alloy powder.
2. Description of the Prior Art
One of the motivations for the development of the invention herein disclosed was the need for high hardness, wear resistant and low resistivity electrical contacts. Because of the problems and costs associated with the current technology, new approaches were considered and as a consequence of the research the present invention was discovered. It is understood that there are more applications for the products and the processes of the present invention than the making of useful and superior electrical contacts; however, electrical contacts and the problems associated with the present state of the art are used in discussing the prior art.
Alloys based on the noble metals are important in forming low-energy electrical contacts in modern electrical systems. The noble metals resist oxidation and corrosion, while exhibiting high electrical conductivities. Because of this combination of properties, the noble metals are used routinely in systems containing semiconducting components. Alloys with high contact resistance, upon the closing of a contact would produce or result in voltage surges due to the high initial transient resistance which alloys having high contact resistance would possess. Such surges are fatal to semiconductors. Thus, there is a natural marriage of semiconducting materials and noble metals used for interconnections. For these reasons the noble metal alloys are used in potentiometer contacts, sliding contacts, commutators, circuit probes, slip rings, make-and-break contacts, and various relays or switches.
Over the years, the needed properties of noble metal low-energy electrical contact alloys have been established. Important requirements are a low contact resistance, resistance to oxidation, tarnish, and polymerization of organic vapors, high electrical and thermal conductivity, high elastic modulus, high strength, and wear resistance. Most of these properties are easily attained with noble metal alloys, but wear resistance requires a high alloy hardness. Since the noble metals exhibit ordering phases, it is common to rely on alloying additions that promote a high hardness through precipitation or ordering. Because of cost, and these other criteria related to the desireable properties of the material, the usual alloy formulation relies on mixtures of common high conductivity metals. One of the most successful alloy groups is located near the center of the palladium-silver-copper ternary. These alloys have sufficient nobility to protect against tarnish and corrosion in most industrial atmospheres. The mechanical properties of the alloys can be altered over a considerable range depending on the degree of ordering induced through heat treatments. Thus, component fabrication is aided by the low strength and high ductility found in the disordered state, and wear resistance in service is aided by the high strength found in the ordered state.
There are factors associated with noble metal alloys that prove to be a continual source of problems. First is the cost of the raw material. The manufacturing sequence to form a final product from ingot material requires time, and with expensive material there is a desire for rapid inventory turnover. Since fabrication involves waste and recycling, net-shape forming approaches, such as powder metallurgy, have merit in rapid product fabrication with minimal waste. Furthermore, the palladium-silver-copper alloys exhibit high work hardening rates. The traditional metalworking techniques involved in fabrication of contacts require a large number of long anneals to eliminate the work hardening. Consequently, compromises in component design, alloying, and product performance occur in order to minimize manufacturing problems.
The following patents are representative of the developments in recent years. Note that none of the patents discussed anticipate the alloy powder of the instant invention nor do they have the particular characteristics herein described. Clearly, Pd-Cu-Ag alloys are well known and are said to be useful for electrical contacts. For example, see U.S. Pat. Nos. 2,187,378 and 4,149,883.
The Japanese appear to be particularly active in this field. Note from the abstracts of the Japanese patents that Pd-Cu-Ag alloys are known for various uses including use as an electrical contact material. However, none of the abstracts disclose the particular alloy defined and described herein.
The abstract of Japanese Pat. No. 52-47516 discloses an electrical contact alloy containing 30-50% Pd; 10-50% Ag and 10-55% Cu. An alloy for use as an electrical contact containing 5-30% Ag, 5-30% Cu and 50-95% Pd is described in Japanese Pat. No. 53-48168.
The abstract of Japanese Pat. Nos. 59-107048; 59-107049 and 59-107050 disclose slide contact material containing 30-50% Pd; 20-40% Ag and 20-40% Cu. An additional ingredient is added to each of the alloys described in these three Japanese patents. In 59-107048 the additional ingredient may be boron. Note however, that there is no suggestion to add boron and phosphorous to the composition or to select the more particular amount of Pd-Ag and Cu used in the instant invention. Even more importantly, it should be noted that the use of the metalloids such as boron and/or phosphorous in this prior art and others is as a deoxidizer and as such the amounts of the metalloids is almost an order of magnitude less than the amount taught and claimed herein. In fact if too much metalloid is used in the process where deoxidation is desired the process of making the alloy is hindered. What is being taught and claimed in the present specification is antithetical to the teaching of the prior art. In the instant invention it is desireable to have the lower melting temperature as an aid in the supersolidus sintering process. In addition, there is no teaching in these three patent abstracts to form the alloy into a powder having the particular grain size and melting characteristics which make it particularly suitable to supersolidus sintering techniques.
U.S. Pat. No. 1,935,897 and British Pat. No. 354,216 disclose a Pd-Cu-Ag alloy which may contain a metalloid deoxidizer such as boron. The inclusion of phosphorous in addition to boron is not suggested. There is also no suggestion for selecting the particular range of ingredients nor of forming the alloy into a powder having the characteristic grain size and melting properties found in the alloy powder of the present invention.
The application of powder metallurgy to noble metal electrical contact alloys provides a possible solution to several problems. First, the economic factors of rapid inventory turnover, minimized scrap, and easy material recycling are very favorable. Second, the reduced number of manufacturing steps greatly aids productivity, since direct shaping is possible. This decreases the time that valuable material is in processing and decreases the inventory control problems inherent with precious metals. Third, the benefits of new compositional possibilities cannot be overlooked. Since deformation is not needed and work hardening is not a concern, new high hardness compositions can be processed by powder metallurgy. Previously, these compositions were unavailable because of processing constraints.
The development of new alloys with new processing approaches is the object of this invention. There has been combined, rapid solidification technology in powder fabrication with metalloid alloying to produce a new class of palladium-based low-energy electrical contact alloys. The metalloid additions greatly increase the hardness and aid sintering densification. Since net shape forming is an obvious component of this research, the modified alloys provide benefits and/or desireable characteristics. The instant invention opens up a wealth of opportunities in forming complex shapes out of coarse powder. The use of the coarse powder from the compositions of the present invention may indeed be ideal for full density processing by injection molding and supersolidus sintering (which is revolutionary because to the present time it has been assumed that only fine powders of 10 micrometers or less were useful in injection molding. The fine powder is a major problem, because of the long debinding time associated with the fine particle size. Many are seeking a solution to this problem.). The advantages of the present invention include lower powder fabrication costs since traditional atomization technologies will be suitable, easier handling and molding with the coarse particle size, faster processing because of rapid debinding and sintering, and better properties because of the homogeneity of the input powder.